164 ZOOLOGY 



have permanently grown together. The reflection of the 

 mantle seems to he of advantage by affording additional 



protection to the visceral mass. 

 But after the complete over- 

 growth of the mantle the shell 

 seems to be useless, and con- 

 sequently degenerates. 



The Pulmonata are either 

 terrestrial, like the slug, or 

 aquatic. Of the terrestrial 



Fia. 150. -Shell of iielix alboia- pulmonates other than the 



bris, a cQmmon forest snail. Nat. , s \ U cr the most important are 

 size. Photo, by W. H. C. P. ., , , , TT T \ 



the snails of the genus Helix. 1 



Helix 2 is noteworthy, because it is richer in species than 

 any other inolluscan genus, since it contains over three 

 thousand species. The distribution of the genus is world- 

 wide. In North America the snails are most abundant in 

 limestone regions, consequently they are more numerous 

 in individuals in the South and West than in granitic 

 New England. One of the most interesting species is 

 Helix nemoralis, a European form, which has been intro- 

 duced into our country at Burlington, New Jersey, and 

 Lexington, Virginia. At these places it has multiplied 

 so rapidly, and varied to such a degree, that three hundred 

 and eighty-five varieties have been enumerated from an 

 area, at Lexington, not over one thousand feet in extent 

 in its greatest diameter 3 (Fig. 151). Any species of 

 Helix collected in large numbers is apt to show abnor- 

 malities in the number of tentacles and of eyes upon the 



1 Xi, a turning round, as in spire of snail shell. 2 Fig. 150. 



3 See paper by Professor J. L. Howe, in American Naturalist, Decem- 

 ber, 1898. 



